1. Field of Invention
The present invention relates to organic electroluminescent devices, which are electrical device light emitting devices used for displays, light sources for displays, and the like, and to manufacturing methods therefor.
2. Description of Related Art
Recently, as self-luminous displays replace liquid crystal displays, development has been advancing rapidly of light-emitting devices (organic electroluminescent devices, hereinafter referred to as “organic EL devices”), which have a structure in which a light-emitting layer formed of an organic material is provided between an anode and a cathode. Among these, a high-transmission EL device, a so-called transparent EL device (TOELD) used in the visible light region, which can emit light from the two electrode sides, has been desired, since overlapped displays can be performed by disposing another display device thereunder. A particular structure of this device is disclosed in, for example, Appl. Phys. Lett. 68(19), 6 May 1996, p 2606. In this publication, it is disclosed that an aluminum complex Alq3, which is a low molecular material, is used as a light-emitting layer, a cathode is formed by co-deposition of Mg and Ag, an ITO film is formed by sputtering thereon for sealing or for assistance to the cathode so as to form a device, and a threshold voltage of approximately 8 V is achieved. In the structure described above, in view of the life and the threshold characteristics of a material used for the light-emitting layer, Mg and Ag are used for the cathode, and in addition, ITO is used for the upper layer thereon.
Concerning organic EL devices, a light-emitting material having a low threshold value can be used for a light-emitting layer, and a metal material having a low work function can be used for a cathode so as to realize operation at a low threshold voltage. However, in the structure proposed in the paper described above, Mg and Ag are not sufficient in view of the work functions, and since ITO is additionally deposited on the metal material by sputtering, in order to prevent the degradation thereof, Mg is oxidized, whereby a problem may arise in that an increase in threshold voltage of the device cannot be finally avoided.